The objectives of this research are to delineate the roles of the DNA polymerase and the 3' to 5' exonuclease activities of DNA polymerase I in maintaining the fidelity of DNA synthesis. The accuracy of nucleotide selection by the polymerase activity and the efficiency with which an incorrectly incorporated nucleotide is excised by the proof-reading 3' to 5' exonuclease activity will be investigated. In these studies we will take advantage of our recent discoveries that both the polymerase and 3' to 5' exonuclease activities can be selectively inhibited: the 3' to 5' exonuclease activity by nucleoside 5'-monophosphates and polymerase activity by 1,10-phenanthroline. We will also study the mechanisms of mutagenesis induced by base analogs, divalent cations and UV light. The mechanism of template-directed polymerization and proof-reading, as well as the interrelationships among the DNA polymerase, 3' to 5' exonuclease and pyrophosphorylase activities will also be investigated by kinetic analysis, inhibitor studies and studies on factors which influence the stability of base pairs.